Electricity consumption
E (kWh) = P (W) × t (h) / 1000
How much an appliance uses, and what it costs: E = P × t, the power in watts times the hours of use, returned in kilowatt-hours then in money at your contract’s price. The tool gives the day, the chosen period and the yearly projection, refuses more than 24 hours per day, and recalls the two truths nameplates keep quiet: the displayed power is a maximum, not an average, and standby is about duration, 5 W around the clock weighing more than a big occasional appliance.
6
- Calculation
- E = P × t = 100 W × 10 h / 1000 = 1 kWh per day
- Energy over the period
- 30 kWh
The appliance uses 30 kWh over the period, that is 6; at this pace, 73 per year.
The kilowatt-hour is an energy, not a power: 1000 W for one hour, or 100 W for ten hours, the same thing. It is what the meter counts and the supplier bills; the "kW" of a subscription is something else, a maximum subscribed power.
The nameplate power is a maximum, not an average: a "150 W" fridge only draws that with the compressor running, a "3000 W" oven modulates after heat-up. For a cycling appliance, the honest measurement comes from a plug-in power meter or the energy label (kWh/year), not from the plate.
Standby is about duration, not power: 5 W around the clock make 44 kWh per year, more than a weekly washing machine. Multiplying a small power by 8760 hours is the most profitable calculation on this page.
Scientific dossier
What the tool computes, what it assumes, where it stops being valid, and where its data comes from.
Method & formulasE (kWh) = P (W) × t (h) / 1000
E (kWh) = P (W) × t (h) / 1000
cost = E × price per kWh
yearly projection = E per day × 365
The kilowatt-hour is the energy of one kilowatt during one hour: one hundred watts for ten hours make exactly one kilowatt-hour, and that is the very definition of the unit. The cost follows in one multiplication: the meter does nothing else.
- Kilowatt-hour (kWh)
- · the energy unit of the bill: one kilowatt held for one hour. Equals 3.6 megajoules.
- Nameplate power
- · the maximum the appliance can draw, written underneath. An appliance that cycles or modulates uses less on average.
- Standby
- · the small permanent power of an "off" appliance. Small in watts, large over the 8760 hours of the year.
- Subscribed power
- · the "kVA" of the subscription: an instantaneous ceiling, not a consumption. Confusing it with kWh is the classic bill-reading mistake.
Validity domainThe calculation assumes a constant power during the declared hours: exact for a resistor or a lamp, optimistic for anything that cycles or modulates.
The calculation assumes a constant power during the declared hours: exact for a resistor or a lamp, optimistic for anything that cycles or modulates. The price per kWh is the one you enter: the tool embeds no tariff, because they change by country, by contract and by hour of the day in dynamic-pricing offers. Off-peak hours are handled as two calculations, one per tariff.
Reading the resultThe yearly projection is the line that decides: 30 a year rarely justifies replacing an appliance, 300 does.
The yearly projection is the line that decides: 30 a year rarely justifies replacing an appliance, 300 does. For appliances that cycle (refrigeration, thermostated heating, ovens), the nameplate overestimates: the energy label gives kWh/year measured in standard conditions, and a plug-in power meter gives yours. For frank-power appliances (kettle, hair dryer, lighting), the plate and this calculation land right.